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Impacts
of Climate Change
on Birds

ost
of the birds that breed in Atlantic Canada are migratory, spending
brief summers here and migrating through the rapidly-changing weather
of spring and autumn. They winter far away from the harsh cold endured
by more hardy resident species such as chickadees and jays.

(Photo: Dorothy McFarlane)

Black-capped
chickadee, adapted to harsh Canadian winters

Climate affects where birds live (their geographical
distribution); most species occupy a limited range of the earth's
surface. Migratory species occupy several different areas of the
planet in the course of their lives, while residents are confined to a
much narrower range of climatic conditions. The climates experienced
by birds following these alternative strategies are of course quite
different, and so are the adaptations needed to survive and reproduce.

Birds are affected directly by climate, for example through their
tolerance for a range of temperature or rainfall. They are also
affected indirectly, through the effects of climate on their habitat
and food supply. It is often difficult to separate direct and indirect
effects, but in the case of North American songbirds, we know that
about half the winter distribution is determined by the availability
of suitable habitat and half by January temperatures. Thus as winters
become warmer, bird distributions are likely to change, but only as
fast as their habitats can respond to the changing climate. So to
fully understand or predict how birds will respond to climate change,
we need to study not only how they themselves respond to climate, but
how the plants and other organisms that make up their habitats will
respond. For example, if our climate becomes unsuitable for Trembling
Aspens (as some have suggested) the woodpeckers, chickadees,
nuthatches and other cavity-nesting birds for which this species
provides most nest sites will either have to find an equally suitable
species to nest in, or will decline in numbers.

Bicknell's Thrush, a threatened species that nests in
high-altitude forest in north-eastern U.S., southern Quebec, northern
New Brunswick and Cape Breton, may lose much of its habitat if warming
reduces available high altitude coniferous forest and birds can't
successfully move further north to similar habitats. This species
winters in a very few locations on Caribbean islands where its habitat
is at risk from deforestation and the spread of agriculture into
mountain forests.

(photo: Dan Busby)

Bicknell's
thrush, a high-altitude species at risk

Many of the changes we expect to see in birds as a result of
changing climate are quite subtle. One of the commonest seen in other
parts of the world is for birds to start nesting earlier; in many
species it seems that the signal to lay the first egg is one of
temperature, so if springs become warmer, birds breed earlier. In
North America, Tree Swallows show signs of this effect, breeding on
average about 9 days earlier now that they did 30 years ago (though
this effect is not seen in Atlantic Canada). In Britain, many species
show this effect, in some cases nesting as much as 18 days earlier
than 30 or 40 years ago. Birds use temperature as a cue to begin
nesting partly because it is often a good predictor of the timing of
food supplies for the young. Most small birds feed their young on
insects (even if the adults feed on seeds or berries), and need to
time their egg laying so that the chicks hatch out when insects are
abundant. If the emergence of insects does not respond to the same
temperature cues as the birds do, then their nesting may become mis-timed
in relation to their food supply, and they will breed less
successfully until they can make the appropriate evolutionary
adjustment to their timing of breeding.

Migration is a critical phase of the life-cycle in many species.
One of the most important local examples is the Semi-palmated
Sandpiper, which breeds in the Arctic and winters in northern South
America; en route, most of the world's population of this species
migrates through the upper Bay of Fundy in late July and early August.
There they fatten up for the long flight non-stop across the western
Atlantic Ocean. To make this flight successfully, they need to take
off with a tail wind, and the weather systems that produce such winds
must occur at the right time to coincide with the availability of
their food (mud-shrimps) and the stage of the sandpipers' annual
cycle. Their nesting grounds (arctic tundra), wintering grounds
(tropical mudflats) and migration habitat (temperate mudflats) are all
susceptible to the rise in sea level that is already occurring as a
result of climate change. This is a species about which we know quite
a lot; there are many others about which we know much less but which
are presumably equally vulnerable to a variety of aspects of climate
change.

Some birds will undoubtedly benefit from changing climates. Recent
range expansions into our region of Mourning Doves and House Finches,
for example, may be attributable at least in part to the warmer
winters of the 1990s. Birds have been adapting to changing climates
throughout their long evolutionary history, and will no doubt continue
to do so. Yet current and predicted climate changes differ in two
important ways from earlier, natural ones; they are much faster, and
they are taking place in addition to a battery of other environmental
changes we have inflicted on birds and the rest of the world's
biodiversity, such as habitat loss and fragmentation, and chemical
contamination (including pesticides and pollution). The combined
effects of these manmade onslaughts on natural systems may prove too
much for some of their habitats.